12 THK XKHVOIS SVSTKM AM) ITS CONSKKY. \TION 



this class, and very much as the skeleton determines the 

 general form of the body, so cadi origin is to be thought 

 of as possessing a minor framework of material which 

 behaves for the most part as though it were not really 

 alive. 



It is usual to place the tissues in four large groups. 

 Besides tin' connective, to which reference has just been 

 made, we speak of the epithelial, the muscular or con- 

 tractile, and the nervous. Epithelial tixxncx are those 

 which form surface layers, either internal or external. 

 They arc the linings and the coverings of the organs. 

 The skin, or at least its superficial portion, is an obvious 

 example. While all the epithelia answer to the descrip- 

 tion of "coverings or linings," they vary widely in other 

 respects. Under the general term are included structures 

 as unlike as the calloused sole of the foot, the exquisitely 

 clear cornea of the eye, and the excessively thin parti- 

 tions interposed between the blood and the air in the 

 lungs. Judging the epithelial tissues by their share in 

 the total of the life-processes, we may >ay that they have 

 both passive and active parts to perform. In some places 

 their function is merely protective; in others they have 

 well-marked chemical activity. Secretion and absorption 

 are epithelial duties, and neither the one nor the other 

 could be carried out in its fulness by membranes not 

 living. 



Citntnirlilr //.s-Nj/r* are those which originate the move- 

 ments of the parts of the body. Organi/ed into large 

 masses, they form the familiar muscles. The heart is 

 mainly composed of a peculiar contractile material 

 which is responsible for its beating. The same property 

 is manifested by elements found in the walls of the ali- 

 mentary canal, the blood-veeU. the urinary bladder, 

 the uterus, and other hollow viscera. It is evident that 

 the contractile tissues are eminently "alive" that no 

 lifeless substances could be expected to behave as they 

 do. Three principal varieties are recogni/ed, conveniently 

 distinguished as the skeletal, the cardiac, and the visceral 



